I see there's a "Low GGA" thread that's fairly recent, how about a "High GGA" to go with it?

We have used a well-mixed primary effluent composite sample as seed for many years. Since we always ran a PE BOD for process control we didn't have to set up separate seed control bottles and our GGA values were always acceptable. We use from 1.5 to 3.0 ml of seed depending on the season and our SCF is almost always 0.6 - 1.0 mg/L. We use Walmart distilled water and blanks are good with depletions typically <0.1 mg/L. We used NCL standard for many years and switched to Hach's EZ GGA ampuoles in early 2009 thinking it would be easier to use and possibly more accurate and stable.

We were pretty consistent for years with a positive bias of around 10 mg/L through 2009 but beginning last year our GGA rose to ~215 mg/L and since May our GGA has been running in the mid-220s. Finally last week we had invalid BODs with GGAs of 233 and 234 mg/L. We switched to settled influent (24HC) for seed last week and also put up a GGA with our usual PE seed. The first of those two tests will be completed tomorrow.

What are some possible causes of high GGA bias with good blanks and SCF in the proper range?

I don't understand how running a PE sample in each batch substitutes for running a seed control bottle. The only valid way to come up with a seed control factor (SCF) is to run a seed control bottle (or 2 or 3) and determine how much depletion you get from only your seed. Perhaps if you explain how you justify omitting the seed control bottle when the method requires it we can come up with a clue as to what might be causing your higher GGAs.

We run primary effluent BOD twice a week for process control. That same sample serves as seed material for effluent and GGA bottles, therefore the SCF is calculated from the depletions in the PE BOD bottles. If we used the supernatant from the PE sample we would have to run seed control bottles but we avoid running a separate series by using well-mixed samples for both seed and PE BOD.

It is possible that your Primary effluent (PE) sample now contains nitrifiers which were not in the sample before. Maybe the influent changed, pumping the primary differently, or an additional recycle stream has contributed nitrifiers in the PE that is used for seeding and now ammonia is being oxidized.

Purchase a seed material that you know without nitrifiers and compare results with the PE seed and if the results of the GGA are lower then the PE seed probably contains some nitrifiers.

Alternately, set the PE sample with and without nitrification inhibitor, BOD and CBOD, to see what effect nitrification has on results.

We ran two sets of BODs last week with two GGAs per set, one with PE and another with settled influent. The range was 227 to 233 mg/L with the PE acceptable in the first set and the influent acceptable in the second set. This week we ran three tests and used PE in half the GGAs and samples and Polyseed in the balance of the samples and the second GGA. Those bottles will be out starting Sunday.

Is there a way to confirm presence/absence of nitrifiers in our PE? Will conductivity analysis help to determine the quality of our Wally water?

Aha! I was interpreting "PE" as "performance evaluation" and not "primary effluent".

In reply to your questions "Is there a way to
confirm presence/absence of nitrifiers in our PE? Will conductivity
analysis help to determine the quality of our Wally water?", you can run a BOD and a CBOD on the primary effluent. If there is no substantial difference in the two results, it could mean that their are an insignificant number of nitrifiers in the sample (the inhibitor in the CBOD has nothing to inhibit), but it could also mean your inhibitor isn't working.

Running a conductivity check on your source water will not rule out the presence of organic material. The best check of your source water (as well as dilution water) is the blank. You say blanks are normally 0.1 mg/L or less which meets method criteria, but I would be wondering what is causing the 0.1 mg/L depletion. If you sometimes get a negative blank (i.e., your DO level appears to increase over the 5-day incubation), it could be simply random error causing those 0.1s. But if the blanks are essentially always positive there is a systematic problem (i.e., bias). That bias would cause your GGA results to be biased on the high side by approximately 5 mg/L (assuming your dilution factor for the GGA is ~50). I don't know if "Wally" sells it, but you might be able to find a store that carries "steam distilled" distilled water. Look for prominent words "STEAM DISTILLED" on the bottle as opposed to a small print statement that says something like "this water goes through a steam distillation process".

Perry...a 5 mg/L high bias on our GGAs from our 0.1 mg/L blank depletions surprised me. I had never bothered to do the math.

Our Walmart water says it is steam-distilled in small print. I don't think our blanks have been any worse since we swithched from Poland Spring (large print steam-distilled) a couple of years ago. Walmart is half the price and our Poland Spring distributor stopped carrying it.

Our GGA is back in a normal range with values from 202 to 210 this week using primary effluent in half the bottles and polyseed in the others.

What can be some other sources of high GGA? My GGAs have been right around 200 for a year, but within the last month they are running on the high side (last week was 222). I also run a QC row, and they have been running high (130 for a 110 standard). I use distilled water and polyseed. My blanks have had 0 depletion, and my air calibrations have been close to theoretical value.

Jesicca, it's too early to tell if you have a problem, and if you do, it's one that many analysts would love to have. Polyseed usually gives low GGAs because it has no nitrifying bacteria meaning you are essentially running a carbonaceous BOD (CBOD).

You didn't say anything about your precision (as indicated by the standard deviation of repeated analyses of the GGA standard). If your precision is good, that coupled with the good blanks you mention, makes it most likely that the increase has something to do with the seed. Did you start using a new bottle of Polyseed at about the same time this GGA increase occurred? Did you change the prep procedure in any way? Any other changes in the lab (e.g., new incubator, temperature change, new equipment)?

My advice is to continue what you have been doing for the last month and see if the "problem" (to you...I don't see it as a problem yet) persists. If you are not already using a control chart, I would suggest you start using one...it helps visually tell what is happening to bias (trends upward or downward from the previous average) and precision (change in the spread of results from one batch to the next). I would be glad to send you an Excel® program for constructing/using a control chart.

The last sentence in your posting puzzles me. The purpose of doing a calibration is to make the meter/probe agree exactly with the theoretical value. When using a chart to determine what the theoretical value is, be sure the chart takes into account both temperature andreal-time atmospheric pressure (not a fixed pressure based on the elevation of your lab). Such a chart can be found at http://www.perrybrake.com/BODSolutions.html.

And one final thought. Unless someone is requiring you to run the 100 mg/L standard, I would drop it. Why run two standards, GGA and the 100 mg/L. The only valid reason for doing that is to run a standard that is closer to one of your plant samples, but 100 mg/L sounds too low for an influent and too high for an effluent.

A little advice requested. For the last few months we have been having our GGA's come out in the 220-230 mg/l range. We have tried switching to the EZ GGA's but they come out even higher! We use a settled influent grab that has been filtered for our seed. We have tried polyseed many times, but that gives us LOW GGA's. Blanks are fine. We have been trying running an extra row of GGA's without the nitrification inhibitor and they are coming out right around 200. Did I just get a "hot" lot of TCMP? Could I just skip adding the TCMP to the GGA's?